Electrodeless high intensity discharge (HID)lamps have been described extensively in the prior art. In general, electrodeless HID lamps include an electrodeless lamp capsule containing a volatilizable fill material and a starting gas. The lamp capsule is mounted in a fixture which is designed for coupling high frequency power to the lamp capsule. The high frequency power produces a light-emitting plasma discharge within the lamp capsule. Recent advances in the application of microwave power to lamp capsules operating in the tens of watts range are disclosed in U.S. Pat. No. 5,070,277 issued Dec. 3, 1991 to Lapatovich; U.S. Pat. No. 5,113,121 issued May 12, 1992 to Lapatovich et al.; U.S. Pat. No. 5,130,612 issued Jul. 14, 1992 to Lapatovich et al.; U.S. Pat. No. 5,144,206 issued Sep. 1, 1992 to Butler et al.; and U.S. Pat. No. 5,241,246 issued Aug. 31, 1993 to Lapatovich et al. As a result, compact electrodeless HID lamps and associated applicators have become practical.
The above patents disclose small cylindrical lamp capsules wherein high frequency energy is coupled to opposite ends of the lamp capsule with a 180.degree. phase shift. The applied electric field is generally colinear with the axis of the lamp capsule and produces a substantially linear discharge within the lamp capsule. The fixture for coupling high frequency energy to the lamp capsule typically includes a planar transmission line, such as a microstrip transmission line, with electric field applicators, such as helices, cups or loops, positioned at opposite ends of the lamp capsule. The microstrip transmission line couples high frequency power to the electric field applicators with a 180.degree. phase shift. The lamp capsule is typically positioned in a gap in the substrate of the microstrip transmission line and is displaced above the plane of the substrate by a few millimeters so that the axis of the lamp capsule is colinear with the axes of the field applicators.
The electrodeless HID lamps disclosed in the prior art provide highly satisfactory performance. However, in some cases, arc bowing and overheating of the lamp capsule wall have been observed. In extreme cases, the discharge within the lamp capsule has extinguished when coming in contact with the lamp capsule wall. In other cases, overheating has caused the lamp to soften and bulge. Such operation reduces the operating life of the lamp capsule and limits the power level which can be applied to the lamp capsule.